This invention relates to a monitoring apparatus for monitoring digital equipment in a digital transmission system, and more particularly to digital equipment which is a combination of a digital multiplexer and a digital demultiplexer. The digital equipment therefore multiplexes a plurality of input signals of a first transmission rate to a single or multiplexed output signal of a second transmission rate. The single output signal is transmitted to a counterpart digital equipment and demultiplexed into reproductions of the above-mentioned plurality of input signals. In contrast to the counterpart digital equipment, the first-mentioned digital equipment will be called a local digital equipment. A like single output signal is supplied from the counterpart digital equipment to the local digital equipment as a single or multiplexed input signal. The local digital equipment demultiplexes the single input signal into a plurality of output signals. Such digital equipment is described, for example, in an article contributed by Tomonori Okada et al to "NEC Research and Development," No. 78 (July 1985), pages 48 to 60, under the title of "MX3 Digital Multiplex Equipment," together with monitoring function for the equipment.
In the manner known in the art, each of the above-mentioned plurality of input signals is timed by a synchronization signal which will be referred to herein as an original synchronization signal and has a predetermined pattern indicative of the above-mentioned first transmission rate. It is therefore possible to understand that each input signal includes an original synchronization signal of a predetermined pattern. Similarly, the single input signal and each of the plurality of output signals include an input synchronization signal of an input pattern and an output synchronization signal of an output pattern, respectively.
Such a digital equipment usually comprises on-line and off-line multiplexers and on-line and off-line demultiplexers. The off-line multiplexer and the off-line demultiplexer may be substituted for the on-line multiplexer and the on-line demultiplexer, respectively, when failure or malfunction occurs in the on-line multiplexer and demultiplexer. Therefore, the digital equipment is protected from failure and malfunction and has a protection function. With the digital equipment, each of the on-line and the off-line multiplexers and the on-line and the off-line demultiplexers should be monitored so as to detect presence or absence of failure or malfunction in each of the multiplexers and demultiplexers. To this end, a monitoring apparatus is used in combination with the multiplexers and the demultiplexers.
As will later be described more in detail, a conventional monitoring apparatus supplies a monitoring signal to the digital equipment so as to monitor the off-line multiplexer and the off-line demultiplexer. The monitoring signal has a specific pattern irrespective of the plurality of input signals and the single input signal which are given to the on-line multiplexer and the on-line demultiplexer, respectively. The monitoring signal is successively circulated through the off-line multiplexer and the off-line demultiplexer and returned as a circulated signal back to the monitoring apparatus. The circulated signal is compared with the monitoring signal to detect occurrence of failure.
On the other hand, the on-line multiplexer is monitored by sending the plurality of input signals back to the monitoring apparatus on the one hand as first comparison signals, by allowing the plurality of input signals to pass through the off-line multiplexer and the off-line demultiplexer as second comparison signals, and by comparing the first and the second comparison signals in the monitoring apparatus.
Furthermore, the on-line demultiplexer is monitored by delivering, to the monitoring apparatus, the plurality of output signals resulting from the single input signal as third comparison signals, by delivering the single input signal to the off-line demultiplexer to allow the delivered single input signal to pass through the off-line demultiplexer as fourth comparison signals, and by comparing the third and the fourth comparison signals.
With this system, each of the multiplexers and the demultiplexers inevitably becomes complex in structure because a superfluous selector should be included in each of the multiplexers and the demultiplexers to send the first comparison signals back to the monitoring apparatus or to deliver the third and fourth comparison signals to the monitoring apparatus. In addition, the monitoring apparatus should comprise two comparators one of which compares the first and the second comparison signals and the other of which compares the third and the fourth comparison signals. Therefore, the monitoring apparatus also becomes complex in structure and becomes bulky in size. Such complex multiplexers and demultiplexers need complicated control of each of the multiplexers, the demultiplexers, and the monitoring apparatus on monitoring each of the multiplexers and the demultiplexers.